Epidemiological and experimental data indicate that both vitamin D and calcium exert anti-neoplastic effects in the large bowel. However, evidence from clinical trials is required to clarify the synergistic and independent chemo preventive effects of these two agents. Neoplastic polyps (adenomas) of the bowel are recognized pre-malignant lesions and effective endpoints for chemoprevention studies. Our research consortium has successfully conducted several chemoprevention trials using these endpoints, and in this resubmission, we propose a multi-centered randomized, double blind, partial factorial trial of the efficacy of calcium carbonate and vitamin D in preventing the recurrence of neoplastic polyps of the large bowel. The study will involve 10 clinical centers in North America, a pathology center and a coordinating center. Each subject will have had at least one large bowel adenoma removed in the 4 months prior to study entry, with complete colonoscopic visualization of the colorectal mucosa and no known polyps remaining in the bowel. Participants will agree to avoid taking study agents outside the trial and will initially be observed in a 3- month placebo run-in period during which suitability for the study will be assessed. Suitable subjects will be randomized in a 2 x 2 factorial manner to calcium carbonate (1200 mg calcium/day), vitamin D (1000 IU/day), both agents, or placebo. Since many women may decline to forgo calcium supplementation, we will offer women the choice of being randomized to calcium alone or calcium plus vitamin D. Colonoscopic follow-up will occur at either 3 years or 5 years after the qualifying exam, as planned by each subject's physician. As safety measures, serum calcium, creatinine and 25-(OH) vitamin D levels will be obtained at baseline, and 1 year and 3 years after randomization. We anticipate enrolling about 2456 participants to reach a total of 1966 randomized subjects. Under conservative assumptions regarding event rates and study conduct, we will have greater than 89 percent power to detect a 20 percent reduction in adenoma recurrence with vitamin D versus placebo, and 85 percent power to detect a 25 percent reduction in recurrence with calcium plus vitamin D versus calcium alone.